Charge Conservation in Circuits
The Electric Current Rule (Kirchhoff's First Law)
- The electric current rule is defined as:
The algebraic sum of the currents entering and leaving a junction is equal to zero
- This is a consequence of conservation of charge - current shouldn’t decrease or increase in a circuit when it splits
Series and Parallel
- In a circuit:
- A junction is a point where at least three circuit paths meet
- A branch is a path connecting two junctions
- If a circuit splits into two branches, then the current before the circuit splits should be equal to the current after it has split
- A typical circuit might have a setup where I = I1 + I2+ I3 where:
- I represent the current in the circuit before it branches
- I1, I2 and I3 represent the current in the respective three branches
The current I into the junction is equal to the sum of the currents out of the junction
- The charge is conserved on both sides of the junction
- In a series circuit, the current is the same at any point
The current is the same at all points in a series circuit
- In a parallel circuit, the current divides at the junctions and each branch has a different value.
- The electric current rule applies at each junction
- The sum of the currents before the junction will equal the sum of the currents after the junction
The current divides at each junction in a parallel circuit
Examiner Tip
Junctions only appear in parallel circuits as circuits become more complex. It can be confusing to work out which currents are going into the junction and which are coming out. Drawing arrows on the diagram for the current flow (making sure it’s from positive to negative) at each junction, such as in the worked example, will help with this.
The electric current rule is also known as Kirchhoff's First Law, and you may come across this phrase.
